This invention relates to the reductive coupling of monohydroxyaryl halide compounds, and more particularly, to the isolation and recovery of dihydroxybiaryl compounds formed from such reactions.
Dihydroxybiaryl compounds, including dihydroxybiphenyls, such as 4,4′-dihydroxybiphenyl (also referred to herein as “biphenol”) and 2,2′-dimethyl-4,4′-dihydroxybiphenyl (also referred to herein as “dimethyl biphenol”) have numerous uses in the chemical industry. For example, biphenol can be used in polymer preparation, notably in the preparation of polycarbonates, polysulfones, polyimides, and polyetherimides.
Dihydroxyaryl compounds can be prepared by the reductive coupling of monohydroxyaryl halides, as described in U.S. Pat. No. 5,177,258. Briefly, in the synthesis of biphenol, for example, p-bromophenol is contacted with a reducing agent, such as formic hydrazide or hydrogen, in the presence of an aqueous base, such as sodium hydroxide, and a platinum group catalyst, preferably palladium, which may be supported on carbon. The p-bromophenol is supplied to the reaction with enough base to form the Na-salt, prior to coupling, and after the reaction is complete, to have the biphenol formed exist as the Na-salt. Isolation of biphenol requires protonation of the Na-salt with an acid, such as hydrochloric acid. This is problematic, in part, because introduction of excess acid to the system requires additional processing for its removal after biphenol is formed. Furthermore, when HCl is used for protonation, for example, the Na salt of the acid (NaCl) is formed as a by-product. When produced on a commercial scale, NaCl is expensive to handle and to dispose of.
Thus, improved methods for the isolation of dihydroxybiaryl compounds from their alkali metal salts continue to be sought. In particular, it would be advantageous to avoid the introduction of a foreign acid, such as HCl, to protonate the salt of the dihydroxybiaryl compound. Such a process would be more economical because fewer reagents would need to be purchased and because excess acid would not need to be removed from the system. Furthermore, isolation of the desired dihydroxybiaryl compound would be simplified because the need for additional processing operations to handle the mixture resulting from the use of the foreign acid could be eliminated. It would also be cost-effective in large scale commercial operations if the base represented by the Na salt of the dihydroxybiaryl compound in the product mixture could be recycled in a further reductive coupling reaction of the monohydroxyaryl halide to form the dihydroxybiaryl compound, instead of being neutralized to form water, as is the case when HCl is used.